Interpretive Summary: The rising global atmospheric carbon dioxide (CO2), presently at 360 parts per million, is expected to double within the next century. Predicted climate changes, including shifting rainfall patterns could result in decreased soil moisture in some parts of the world. It is well-known that high CO2 promotes yield, and drought is detrimental to crops, but few tests have been conducted on the combination of these factors. In this study, by scientists at the Agricultural Research Service in Gainesville, FL, rice was grown at ambient or twice-ambient CO2 under continuously flooded conditions or with drought imposed at the critical stage when the seed head was being formed. The purposes were (1) to assess the combined effects of high CO2 and drought on the biochemistry of photosynthesis, and (2) to test if high CO2 could alleviate the adverse effects of drought on leaf photosynthetic capacity. The results showed that high CO2 partially compensated for drought effects on photosynthesis, and especially caused a significant delay of 1 to 2 days in the drought symptoms. The high-CO2 drought plants maintained the essential reactions of photosynthesis during drought that were impaired in the ambient-CO2 plants. For rice, high CO2 provided temporary relief from drought stress.

Technical Abstract:
Rice (Oryza sativa L. cv. IR-72) was grown in sunlit chambers at ambient or twice-ambient carbon dioxide concentration [CO2] under continuous shallow flood or with drainage of the 50-cm depth soil timed to maximize drought at panicle initiation. Photosynthetic rates (Pn) were enhanced by high [CO2] but they were decreased by drought. Decreased evapotranspiration and increased Pn in high CO2 caused a significant delay of 1 to 2 d in drought symptoms and an increase in water-use efficiency. Both high [CO2] and drought induced changes in chloroplast structure and starch granule accumulation, and diminished activities and content of Rubisco. The catalytic turnover rate (Kcat) of Rubisco was not altered by high [CO2], but it was decreased by drought. Inhibitions in activity, protein content, and Kcat of Rubisco by drought were less for high CO2 plants. Severe drought decreased leaf starch, increased leaf sucrose, and resulted in 45% less sucrose phosphate synthase activity (SPS) in low-CO2 plants, but did not affect SPS activity of high-CO2 plants. For rice, high [CO2] partially compensated for and delayed the effects of drought, which allowed these plants to continue photosynthesis, which provided temporary relief from soil drying.